1. Field of the Invention
The present invention relates to remote sensing and more particularly to real-time positioning for geophysical sensing.
2. Description of the Related Art
Remote sensing refers to instrument-based techniques in acquiring and measuring spatially organized data derived from a target scene. Remote sensing includes the reduction according to spectral, spatial or physical properties of an array of target points within a scene representative of features, objects, and materials within the sensed scene. The target points generally are acquired through the deployment of one or more recording devices lacking physical, intimate contact with the item under surveillance. Conventional recording devices include sensors utilizing electromagnetic radiation, force fields, or acoustic energy, and are embodied within cameras, gradiometers and scanners, lasers, radio frequency receivers, radar systems, sonar, thermal devices, seismographs, magnetometers, gravimeters, and scintillometers, to name a few.
Geophysical sensing is a subset of remote sensing and relates specifically to the imaging of subsurface objects. Industrial applications of geophysical sensing include subsurface geological surveying, archeological prospection, hydrocarbon exploration hydrologic studies, oceanographic studies, landmine detection, utility detection, and rebar imaging by way of example. Ground Penetrating Radar (GPR) is a well-known geophysical sensing technique utilizing high frequency pulsed electromagnetic waves (typically from 10 MHz to 1,000 MHz) to acquire subsurface information.
In GPR, electromagnetic waves radiate from a transmitting antenna and travel through material in the target scene at a velocity which is determined primarily by the electrical properties of the material. As the wave energy spreads and travels downward towards the target scene, portions of the wave energy impacting a buried object or boundary with different electrical properties than the surrounding material are reflected or scattered back to the surface while the remaining portion of the wave energy continues to travel downward. The wave energy reflected back to the surface can be captured by a receiving antenna, and recorded for later interpretation.
The most common display of GPR data includes a signal travel time versus amplitude view, and is referred to as a trace. A single GPR trace typically consists of the transmitted energy pulse followed by pulses that are received from reflecting objects or layers. A scan is a trace where a color or gray scale has been applied to the amplitude values. As the antenna moves along a survey traverse, a series of traces or scans are collected at discrete points along the line. These scans are positioned side by side to form a display profile of the subsurface.
GPR is well known for its ability to produce highly resolved, subsurface imagery. Yet, conventional GPR applications cannot exploit the full potential of three dimensional GPR imaging due to overly coarse spatial sampling that occurs during data acquisition. Theory and practice show that full resolution three-dimensional GPR imaging requires the un-aliased recording of dipping reflections and diffractions. For a heterogeneous subsurface, in particular, minimum grid spacing of GPR measurements must be at least a quarter of a GPR wavelength or less in all directions. Consequently, positioning precision must be better than an eighth of a wavelength in order to assure correct grid point assignment. Present positioning technologies applied to GPR applications fail to achieve the requisite resolution while maintaining a tolerable speed of data acquisition.